Interference eliminating system



j N 11, 1952 COHEN 2,617,927

INTERFERENCE ELIMINATING SYSTEM Filed Nov. 9, 1945.

INVENTOR 1100/15 Coke/7,

ATTORNEY Patented Nov. 11, 1952 2,617,927 ICE INTERFERENCE ELIMINATING S YS TEM Louis Cohen, Bethesda, Md.; Louise Sissman, ad-

ministratrix of the estate of said Louis Cohen, deceased, assignor to Louise Sissman as trustee for Louise Sissman, Louis P. Sissman, Joan Louise Sissman, and Peter Louis Sissman Application November 9, 1945, Serial No. 627,675

7 Claims.

This invention relates to the art of radio signal reception. It has for its object to provide improved methods and means for the reception of radio signals, and a particular object is the elimination of interferences from extraneous electrical impulses or discharges of any sort acting on the antenna of the receiving circuit system.

In the reception of radio signals, difliculties are frequently encountered by noises in the receiving system caused by electrical atmospheric discharges generally referred to as static, and noises produced by man made static caused by nearby electric motors, power lines, etc. All these interferences make effective signal reception at times impossible and at best unpleasant, particularly so in broadcast reception where quality of signal reception is highly desirable. 7

By the method described in this application most of the interferences encountered are balanced out and clear signal reception is obtained under conditions which otherwise would render signal reception unsatisfactory and at times a1- together impossible.

Other objects and purposes will appear from the following detailed description of the invention following hereinafter, taken in conjunction with the accompanying drawing illustrating one preferred embodiment thereof.

The drawing shows one embodiment of the invention in which the coupling coil 2 together with the tuned circuit 3, constitute the input circuit of a radio receiver. Terminal II is connectedto the grid and terminal l2 to the filament of the first tube of the receiver. It is preferable that circuit 3, 5 and associated tube be used as a simple radio frequency amplifier. This receiving system employs two antennas, one associated magnetically and the other electro-statically with the tuned input circuit consisting of inductance 3 and capacity 5 of the radio receiver. Antenna I is connected in series with the primary 2 of the transformer 2, 3 and ground 4 constituting one antenna circuit. This antenna circuit is coupled to the tuned input circuit 3, 5 through the mutual inductance between coils 2 and 3. A second antenna 6 is connected through a variable condenser 1 to the input circuit 3, 5 at point 8. It is preferable that the two antennas l and 6 be open-end antennas. The input circuit is grounded at point I 0. Special ground connections at points 4 and H) are not essential; connecting these points to the chassis of the radio receiver in the usual manner is suihcient. The terminals ll, l2 of the input circuit are connected to the grid and filament of first tube of the receiver in the usual manner well understood in the art. By suitably adjusting the inductive coupling 2, 3 in the matter of magnitude and direction and the capacity of variable condenser l, a condition is obtained w by extran o s el ctr cal inte ferences act ing on the two antennas are balanced out against each other in the input circuit 3, 5 without affecting appreciably the responsiveness of the system to the desired signals. But even a loss in efficiency of the circuit system in its responsiveness to the desired signals is not a serious matter if interferences are eliminated. The reduction insignal intensity can be compensated for by additional amplification and even without such additional amplification a weaker signal without interference is an improvement over a stronger signal accompanied by interference from extraneous electrical impulses.

In the operation of the invention it is preferable to maintain the mutual inductance of transformer 2, 3 fixed and obtain all adjustments by varying the capacity of coupling condenser 1. For the broadcasting range of frequencies 550 kc. to 1600 kc. and employing a transformer 2, 3 and condenser 5 normally used for this range of frequencies, I have found that very satisfactory results are obtained when the variable condenser I has a maximum capacity between 10 and 20 mmf. The operation then is very simple: The radio receiver is tuned for the signals desired to be re ceived in the usual manner, and condenser 1 is adjusted for minimum interference. The particular setting of condenser l for interference elimination depends to some degree on the relative lengths of the two antennas I and 6. It is essential to have proper direction of the mutual inductance 2 and 3. Accordingly, if no results in interference reduction are obtained by varying the capacity of condenser l, the connections of either primary coil 2 or secondary'coil 3 should be reversed. Once the correct direction of the mutual inductance between 2 and 3 is established,

no further changes in the transformer connections are ordinarily necessary for any frequency.

It should be pointed out that connecting antenna 6 and condenser l as shown change the tuning of circuit 3, 5 slightly, since the effect of antenna 6 and condenser l is to place a very small additional capacity in parallel with condenser 5. Ordinarily, especially when the circuit 3, 5 is part of a simple radio frequency amplifying stage, this effect may be disregarded. If, however, it is desired to maintain the exact tuning of the first circuit, this may be done by suitably compensating for the additional capacity introduced by antenna 6 and condenser 1. For example, the capacity of condenser 5 may be made smaller and a trimming condenser connected in parallel with it for adjustment, so that as capacity l is increased, the capacity of the trimming condenser is decreased. For simplicity of operation, it may be desirable to have condenser l and the trimming condenser operated by a single control in a manner well known inthe art.

It has been determined experimentally that by the method above described it is possible to substantially eliminate interfering energy from the output of the tube associated with the tuned circuit while leaving a substantial residual of desired signal for further transmission through the radio. Without being bound by this or any other theory of the operation of the invention, it appears to applicant that such operation may be satisfactorily understood from the following:

Every electrical disturbance acts as if charac terized by a frequency of its own, generally of much lower order than the frequencies of radio signals. When the system is properly adjusted for minimum interference, the voltages generated across the condenser by the interfering impulses from antennas l and 6 are of the same character, equal in magnitude, and opposite in direction, thus balancing out. However, under these conditions the voltages generated across the condenser 5 by the desired signals acting on the two antennas will not balance out.

4 p is the differential operator;

We also have the auxiliary equation:

Substituting the value of I7 from (2) into Equations (1), the following three equations are obtained:

Eliminating I1 from the first two equations of Equations (3) ,gives the following:

1 1 1 l I =E lp 7p K 7P EP 2 Eliminating 1'3 from the above two equations gives an expression for Is as follows:

This may be explained on the basis that the distribution between condenser 5 and inductance 3 of the currents produced by voltages generated in antenna 6 is different for low and high frequencies. For low frequencies the current flows almost entirely through inductance 3 and no current flows in condenser 5, while for high frequencies the current flow due to voltages on antenna 6 divides at point 8, part flowing through inductance 3 and part flowing through condenser 5. Because the frequencies of the interfering electrical impulses act as if they are of a much lower order than the frequency of the desired signal, the coupling condenser 1, the inductance coupling 2, 3, or both may be so adjusted as to substantially balance out the interferences without seriously impairing the responsiveness of the receiver to the desired signal.

The following mathematical analysis of the circuit system further amplifies this theory of its operation:

Let E1 and E2 designate the voltages induced in antennas i and 6, respectively. I1, I7, I3, I5 designate the currents in antenna I, antenna 6, coil 3 and condenser 5, respectively. Let C1 desi nate the capacity of antenna l C7 the capacity of antenna 6 in series with condenser 1; L2 the inductance of coil 2; L3 the inductance of coil 3; C5 the capacity of condenser 5; M the mutual inductance between coil 2 and 3. Neglecting the resistances of the various elements of the circuit system to simplify the mathematical discussion, we have the following equations for the voltage distribution in the several branches of the circuit system.

Equation (5) can be readily simplified to the following:

1 C1 4 i 22 i) m 1 5 C1C5p M 1p 5P The above equation may be put in this form:

2. L H i C p C C C C p M c7 0 05p The expression for the voltage across condenser 5 which is applied to the grid of the first tube can be readily obtained by the aid of Equation (7). If I designate this voltage by V5, I have are large quantities of the order of magnitude and of I to ill", and by adjusting C7 a condition can be obtained for balancing out these two large quantities, that is making l ne a V v (IQ E C,

and under this condition the voltage across condenser is, e

d g M2 C 2p+C11p)(C 7+O5+ 7C5p I] 5P For low frequency impulses, say of the order to 10 the term (Lam-M is of the order of magnitude of 10 to 100, a very small quantity compared with the terms M 01 or and hence is affected but slightly by the balance. It is evident from the above mathematical discussion that the circuit arrangement of this invention provides an effective means for balancing out low frequency impulses without appreciably aifecting its efficiency in its response to high frequency signals.

The derivation of the relationship C7 E1 C1 for balancing out interferences obtained in the above mathematical discussion can be also deduced from physical consideration of the circuit system. For a voltage of a low frequency in antenna I, the current generated is determined by the capacity reactance of the antenna circuit, the inductive reactance of coil 2 is relatively small. Hence for antenna I, we have where f is the frequency.

The voltage induced in coil 3 by the mutual inductance M between coils 2 and 3 is,

For antenna 6 if. the voltage is E2, the current .6 7 enerated in it is in a similar way determined by the antenna capacity C7 and is given by This current flows almost entirely through coil 3 and none through condenser 5, because the condenser reactance for low frequencies is very large in comparison with the inductive reactance of coil 3. The voltage developed across the coil 3 between points 8 and H1 is For balancing out the voltages at 8, [0 produced by interfering impulses acting on the two antennas, that is, to make 'V1V :0, we must satisfy the relationship analysis above. It is to be noted that the Formula (13) includes the factor which covers any differences in the voltages induced in the two antennas because of different lengths or differences in proximity to the source of interference.

For high frequencies the conditions are entirely different. The current in antenna I is no longer determined solely by its capacity reactance, the inductive reactance of coil 2 must be included also. For antenna 6, the current at the point B does not flow solely through coil 3. At that point it divides, part flowing through coil 3 and part flowing through condenser 5. So, for the high frequencies we have an entirely different set of conditions and therefore do not balance out as in the case for low frequencies.

The foregoing explanation and mathematical analysis of the theory of operation of my invention has been set forth as a guide to a better understanding of the invention, and is not to be considered a limitation of the scope thereof. Irrespective of the correctness of any theory of its operation, the scope of my invention is as set forth in the appended claims.

The spatial disposition of antennas l and 6 with respect to each other is immaterial to this invention provided they are both exposed more or less in the same degree to the effects of the interfering impulses. Any difference in the amount of interference picked up by the antennas can be compensated for by the adjustment of either of the couplings of the antennas to the input circuit. Also, it is not necessary that the antennas be of the same lengths; this again can be compensated for by the adjustment of the couplings. It is preferable, however, to place the antennas parallel and not widely separated so that at all times they are both exposed in more or less the same degree to the interfering effects.

While I have described my invention as embodied in a specific form and as operating in a specific manner for purpose of illustration, it should be understood that I do not limit my invention thereto, since various modifications will suggest themselves to those skilled in the art without departing from the spirit of my invention, the scope of which is set forth in the annexed claims.

This application is a continuation-in-part of my application Serial No. 544,499, filed July 12, 1944, abandoned.

I claim:

1. In a radio receiving system for energies containing high frequency signal energy and interfering energy having a parallel tuned circuit at the input thereof tuned to the frequency of the desired signal, an antenna circuit including an inductance inductively coupled to said tuned circuit, and a second antenna circuit including a coupling condenser capacitively coupled to said tuned circuit, each antenna circuit including a separate antenna and neither antenna circuit containing a tuned circuit, said antenna circuits transmitting the energies picked up thereby to said tuned circuit in such direction and magnitude to substantially neutralize interfering energy of apparent frequencies different from the frequency of the desired signal energy while permitting further transmission through the radio receiving system .of energy of the desired signal frequency range.

2. A system as set forth in claim 1 wherein the coupling condenser is of the order of .10 to 20 mmf.

3. A system as set forth in claim 1 wherein the antennas of both of said antenna circuits are open-ended antennas disposed adjacent to each other.

4. A system .as set forth in claim 1 wherein the tuned circuit is part of a simple radio frequency amplifying stage and the coupling condenser connected thereto is a variable condenser with a maximum capacity of the order of 10 to 20 mmf.

5. In a radio receiving system for energies containing high frequency signal energy and inter- .fering energy having a parallel tuned circuit at the input thereof, an antenna circuit including an inductance inductively coupled to said tuned circuit, and a second antenna circuit including a coupling condenser capacitively coupled to said tuned circuit, each antenna circuit including a separate antenna and neither antenna circuit containing a tuned circuit, the ratio of the mutual inductance between the first antenna circuit and the tuned circuit coupled therewith t0 the capacity of said second antenna circuit being equivalent to the product of the ratio of the inductance of said tuned circuit to the capacity of said first antenna circuit and the ratio of the voltages induced in the second and first antenna circuits, respectively.

6. A system as set forth in claim 5 wherein the coupling condenser is connected to the high potential side of said tuned circuit.

7. A radio receiving system as set forth in claim 1 wherein the two antennas are open end antennas and are spaced at a small distance from each other and extend in substantial parallelism.

LOUIS COHEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,872,487 Miller Aug. 15, 1932 1,888,278 MacDonald Nov. 22, 1932 1,967,604 Beverage July 24, 1934 2,050,912 Albright Aug. 11, 1936 2,102,401 Yolles Dec. 14, 1937 2,256,233 Berthold Sept. 16, 1941 2,258,283 Feld Oct. 7, 1941 FOREIGN PATENTS Number Country Date 802,392 France Sept. 3, 1936 

